1. Field of the Invention
The present invention relates generally to steelmaking and, more particularly, to steelmaking operations carried out in a twin shell electric arc furnace (EAF). The present invention is particularly suited for use in connection with the EAF steelmaking process, wherein the remelting of scrap is one of the primary sources of raw material.
2. Description of the Prior Art
The EAF steelmaking process has in recent times captured an increasingly larger percentage of the world's steelmaking capacity due to its constantly improving operating efficiencies and production capabilities. Currently, a single EAF operating unit is capable of producing 1,500,000 tons of steel per year. To obtain this annual tonnage, a two vessel or so-called twin shell furnace was developed to maximize utilization of the installed electrical equipment and to minimize fluctuating loads on the utility supply. In the twin shell concept, one furnace shell is powered and in the melting mode while the second furnace shell is non-powered and in one of the tap, charge or fettle (repair) modes. The tap, charge and fettle modes require less time than the melting mode. This non-powered time increment can also be used to preheat a charge of scrap in the non-powered shell. After the scrap is preheated, the non-powered shell is powered to melt the scrap.
Scrap preheating currently takes two forms. One form uses interconnecting ductwork between the two shells with the hot exhaust gases from the powered vessel being drawn through the non-powered unit. This form of preheating is disclosed in co-pending U.S. patent application Ser. No. 08/212,581, filed Mar. 11, 1994, in the name of Raymona J. Burda and assigned to the assignee of the instant application. The other common form of scrap preheating in twin shell EAF processes utilizes a plurality of oxygen-fuel or oxygen/air fuel burners in the shell walls to preheat the charged scrap while the fume or off-gas from each vessel is exhausted separately.
Both of these known preheat practices are effective in improving net unit electrical efficiency and production capacity, but both produce undesirable organic compounds in the spent off-gas exhausted during the scrap preheating cycle. The spent off-gas from the preheating cycle is high enough in temperature to volitize the organic compounds commingled with the scrap. Oftentimes a noxious odor is also produced in the off-gas.
Emissions from an EAF steelmaking process basically occur In the form of dust and gas. Reducing dust emissions to the required environmental levels is, of course, well known in the art, such as by using fabric filters in a conventional bag house, for example. With regard to gaseous emissions, dioxins are currently of critical concern, particularly when heating steel scrap having plastic material such as polyvinyl chloride mixed therewith. It is oftentimes very difficult or impossible to segregate the metal and plastic materials, such as in the case of compacted scrap automobiles for example. Some plastic components will be intimately combined with the steel in the compacted process shape even though magnetic separation is attempted. The term dioxin is used to describe many different organic compounds belonging to the family of polychlorinated dibenzodioxins (PCDD's). Also of concern, from a toxic gaseous emissions standpoint, are furans which belong to the family of polychlorinated dibenzofurans (PCDF's).
In addition, some scrap, such as steel turnings, for example, may contain oil, grease and/or other organic residues, which, upon heating, also cause the emission of volatile organic compounds. While these volatiles are not potentially toxic like the aforementioned dioxins and furans, they may, however, create objectionable odors which are unacceptable in the environment, especially in the vicinity of residential areas.
It is known that PCDD's and PCDF's may be cracked/burned at temperatures above about 1200.degree. C. forming CO/CO.sub.2 and H.sub.2 O as the reaction products. These thermally treated off-gases must then be rapidly cooled in order to avoid a reverse reaction and reformation of PCDD's and PCDF's. It is also known that the non-toxic, noxious smelling, organic compounds resulting from the volatization of oils and greases undergo a comparable decomposition at temperatures on the order of 750.degree.-800.degree. C. which eliminates odor problems.
Heretofore, a commonly employed way of eliminating objectionable organic volatile emissions has involved the use of afterburners to raise the temperature of the off-gases above the critical temperature range. These afterburners combust or crack the organic volatile in an environmentally acceptable manner, but are extremely uneconomical from an energy cost standpoint. The afterburners oftentimes consume as much energy as is saved by the preheating operation, thus, minimizing or offsetting any benefit in energy efficiency resulting from the scrap preheating step.
The present invention solves these aforementioned problems by providing an apparatus and methods for use in a twin shell EAF steelmaking operation which eliminate the emissions of objectionable toxic furans and dioxins, and also noxious smelling, organic compounds as well as carbon monoxide which may be generated during scrap preheating and melting operations. The present invention achieves these desirable environmental objectives in an energy efficient manner.